In this grant, we have the potential to bridge two fields of investigation: innate immune signaling in the nervous system and the homeostatic control of synaptic transmission. The innate immune system is evolutionarily conserved in all animals. Recent work has revealed that innate immune signaling participates in neural development, plasticity and disease. This previous work includes evidence for the function of the C1q component of the complement cascade during synapse elimination, the function of Toll receptors in learning and memory, and the function of NFkB/Rel transcription factors in learning- related plasticity. In this grant, we identify a novel role for innate immune signaling in the nervous system. Specifically, we show that an innate immune receptor (PGRP-LC) and the downstream innate immune signaling cascade (the IMD signaling cascade) are required for homeostatic synaptic plasticity. The receptor appears to function at the presynaptic nerve terminal to induce two downstream effects: 1) the local homeostatic modulation of presynaptic neurotransmitter release and 2) the transcription-dependent maintenance of homeostatic plasticity. Our experiments will explore this possibility in molecular and genetic detail. Our findings may be broadly relevant for our understanding how neural function is stabilized throughout development, during aging and in the context of disease. More specifically, impaired homeostatic plasticity is believed to contribute to diverse neurological diseases including epilepsy, autism spectrum disorders, and anxiety. Our preliminary data and proposed experiments may also document a novel function for an innate immune signaling system that has never before been studied in the nervous system. Thus, our data may be significant at several levels, with implications for our understanding and treatment of diverse neurological diseases.